The present invention relates to an optical device which is used in optical communications, such as wavelength multiplexing communications, and, more particularly, to a wavelength selecting module that selects a light signal having a specific wavelength from a plurality of light signals having different center wavelengths and a wavelength selecting apparatus that selects one or more types of light signals from plural light signals.
Optical devices, such as an optical fiber and collimator lens, have been used in optical communication equipment. As optical communications become more popular, further miniaturization and larger scale integration of optical communication equipment become necessary. Further, optical communications require a technique of selectively demultiplexing light by wavelength. In this respect, optical filters have been proposed, such as an edge filter or a narrow band filter (see FIG. 11), which is a multilayer filter having an alternate stack of dielectric layers with a high refractive index and dielectric layers with a low refractive index.
A filter module 100 according to the related art of FIG. 11 selects a light signal having a specific wavelength from a plurality of light signals having different center wavelengths (xcex1 to xcexn). The filter module 100 includes a two-core collimator 101, a single-core collimator 102, and a multilayer filter 103 provided between the collimators 101 and 102, The two-core collimator 101 includes a two-core capillary 104 which holds two optical fibers, and a collimator lens 105 comprising a rod lens. The single-core collimator 102 includes a single-core capillary 106 which holds a single optical fiber, and a collimator lens 107. Reference numeral xe2x80x9c108xe2x80x9d denotes a sleeve. The filter module 100 passes only a light signal having a specific wavelength (e.g., xcex1) among the light signals having center wavelengths xcex1 to xcexn and reflects the remaining light signals having center wavelengths xcex2 to xcexn.
The specific wavelength (xcex1) is determined in accordance with the wavelength selecting characteristic of the multilayer filter 103 and the filter module 100 always selects a light signal which has the determined specific wavelength. In case of using the filter module 100, therefore, it is not possible to arbitrarily switch between selection of a light signal having a specific wavelength and non-selection of the light signal.
FIG. 12 is a schematic diagram of a wavelength selecting apparatus 200 which has a plurality of filter modules 100A, 100B, 100C and so forth cascade-connected. In the wavelength selecting apparatus 200, the filter module 100A selects a light signal having a center wavelength xcex1 and reflects the other light signals (xcex2 to xcexn). The filter module 100B selects a light signal having a center wavelength xcex2 and reflects the other light signals (xcex3 to xcexn). The filter module 100C selects a light signal having a center wavelength xcex3 and reflects the other light signals (xcex4 to xcexn). Likewise, the light signals that have the center wavelengths xcex4, xcex5, . . . and xcexn are selected one after another.
The wavelength selecting apparatus 200 has the following disadvantages.
(a) Since light signals that have the center wavelengths xcex1, xcex2, xcex3, . . . are always selected by plural (n) filter modules 100A, 100B, 100C, . . . , respectively, it is not possible to arbitrary select more than one type of light signal from light signals of xcex1 to xcexn.
(b) Those light signals which have wavelengths not selected by each filter module are reflected and enter the next filter module. Therefore, a light signal loss occurs every time each filter module reflects light signals and such losses are accumulated in accordance with the number of filter modules. Specifically, as the intensities of the light signals of xcex2 to xcexn fall through reflection by the filter module 100A, the intensity of the light signal of xcex2 that is selected by the filter module 100B is lower than the intensity of the first light signal. Since the intensities of the light signals of xcex3 to xcexn become lower through reflection by the filter module 100B, the intensity of the light signal of xcex3 that is selected by the filter module 100C is lower than the intensity of the light signal of xcex2. Apparently, the intensity of a light signal becomes lower as the selection order of that light signal becomes later.
As the number of filter modules connected becomes larger, the light signal loss (the attenuation of the light intensity) becomes greater. To prevent an increase in light signal loss, it is necessary to improve the connection of the individual filter modules. In case where the intensity of a light signal becomes lower than demanded, the light signal should be amplified by an amplifier. The attempts to improve the connection of the individual filter modules or the use of an amplifier seriously stands in the way of constructing an optical communication system which deals with a vast amount of information.
Accordingly, a first object of the present invention is to provide a wavelength selecting module capable of switching between selection and non-selection of light having a specific wavelength.
A second object of the present invention is to provide a wavelength selecting apparatus which arbitrarily selects one or more types of light signals from plural types light signals having different center wavelengths and suppresses attenuation of the intensity of the selected light.
In a first aspect of the present invention, a wavelength selecting module for selecting a light signal having a specific wavelength from a plurality of light signals having different center wavelengths is provided. The plurality of light signals are provided as diverging light. The module includes a first collimator for collimating the diverging light to generate a collimated light beam and a liquid crystal cell having a predetermined helical direction. The liquid crystal cell separates a light signal having a specific wavelength among the plurality of light signals of the collimated light beam into a left circularly polarized light and a right circularly polarized light, reflects one of the left and right circularly polarized light signals that has a same optical rotatory direction as the predetermined helical direction toward the first collimator in a first state, and passes the plurality of light signals of the collimated light beam in a second state. The liquid crystal cell changes between the first state and the second state in accordance with a change in physical energy applied thereto.
In a second aspect of the present invention, a wavelength selecting apparatus for selecting at least one light signal from a plurality of light signals having different center wavelengths is provided. The plurality of light signals are provided as diverging light. The apparatus includes a plurality of wavelength selecting modules and a plurality of optical fibers for optically connecting the plurality of wavelength selecting modules. Each wavelength selecting module includes a first collimator for collimating the diverging light to a generate a collimated light beam and a liquid crystal cell having a predetermined helical direction and receiving the plurality of light signals of collimated light beam from the first collimator. The liquid crystal cell separates a light signal having an associated wavelength among the plurality of light signals of the collimated light beam into a left circularly polarized light and a right circularly polarized light, reflects one of the left and right circularly polarized light signals that has a same optical rotatory direction as the predetermined helical direction toward the first collimator in a first state, passes the plurality of light signals of the collimated light beam in a second state. The liquid crystal cell changes between the first state and the second state in accordance with a change in physical energy applied thereto.
In a third aspect of the present invention, a wavelength selecting apparatus for selecting at least one light signal from a plurality of light signals having different center wavelengths is provided. The plurality of light signals are provided as diverging light. The apparatus includes a first collimator for collimating the diverging light to generate a collimated light beam and a liquid crystal cell unit for receiving the plurality of light signals of the collimated light beam from the first collimator and reflecting at least one light signal toward the first collimator. The liquid crystal cell unit includes a plurality of stacked liquid crystal cells. Each liquid crystal cell includes a liquid crystal which has a pair of surfaces and a predetermined helical direction. The liquid fix crystal separates a light signal having an associated wavelength among the plurality of light signals of the collimated light beam into a left circularly polarized light and a right circularly polarized light, reflects one of the left and right circularly polarized light signals that has a same optical rotatory direction as the predetermined helical direction toward the first collimator in a first state, passes the plurality of light signals of the collimated light beam in a second state. The liquid crystal changes between the first state and the second state in accordance with a change of a voltage applied thereto. Each liquid crystal cell further includes a pair of transparent electrodes which is provided on the pair of surfaces of the liquid crystal and to which the voltage is applied. At least one liquid crystal enters the first state by individually changing voltages applied to the liquid crystals via the pairs of transparent electrodes.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.